The present invention relates to a method for manufacturing cathode ray tubes used for color television picture tubes and the like. Such tubes are presently fabricated by sealing together a glass faceplate or panel supporting the phosphorescent display screen of the tube, and a glass funnel which supports an electrically conductive interior coating constituting part of the electronic circuitry of the tube. Sealing is accomplished by providing a devitrifiable solder glass at the panel-funnel interface which first flows and then crystallizes during heating to provide a hermetic high-use-temperature seal. Following the sealing together of the panel and funnel to provide a bulb assembly, electronic circuitry is added to the bulb and the bulb is evacuated and hermetically sealed to provide an operational cathode ray tube according to procedures well known in the art.
Conventional tube manufacturing processes typically comprise another heating operation, carried out prior to the sealing together of the glass panel and funnel, wherein the glass panel and phosphoresecent display screen are heated to remove organic display screen components applied during the screen deposition process. This heating operation, referred to as screen baking, is carried out at temperatures near those of the subsequent panel-funnel sealing operation.
For the purpose of energy conservation, it would be desirable to accomplish screen baking and the sealing of the glass panel to the glass funnel in a single sealing step, called a combined bake and seal (CBS) operation. Such an operation would eliminate one thermal cycle and reduce tube fabrication energy requirements accordingly.
It is found, however, that organic vapors evolved from the screen components during heaing chemically reduce the devitrifiable solder glass during a combined bake and seal operation. This chemical reduction, evidenced by a dark discoloration of the devitrified seal at and near the interior walls of the sealed bulb, leads to dielectric breakdown of the seal when high voltages (30 kv or greater) are applied across the tube. Such breakdown can ultimately result in seal failure, loss of vacuum, and failure of the tube.
It has been proposed, for example, in U.S. Pat. No. 3,973,975 to Francel et al., to intermix oxidizing agents with the devitrifiable solder glass to suppress chemical reduction by organic vapors during sealing. However, such procedures can be disadvantageous because the addition of those agents, such as red lead, reduced the dielectric strength of the fired solder glass.
The use of alkali and ammonium sulfates and nitrates as additives to modify the properties of interior conductive funnel coatings is suggested in U.S. Pat. No. 3,947,608 to Duinker et al. However, funnels so coated are thereafter incorporated into cathode ray tube envelopes by conventional processing methods.
It is a principal object of the present invention to provide an improved combined bake and seal process which avoids chemical reduction of the solder glass and thus produces a seal which is resistant to dielectric breakdown and hermetic failure.
Other objects of the invention will become apparent from the following detailed description thereof.